The invention relates to a digital motion picture decoding apparatus and a digital motion picture decoding method for decoding picture data coded by variable length coding for digital storage media, broadcasting, and communication. In particular, it relates to a digital motion picture decoding apparatus and a digital motion picture decoding method capable of correctly decoding and reproducing coded data which is partially discontinuous due to change of program during the reproduction.
Nowadays, demands for general method for coding motion picture and its attached sound are increasing for various applications including digital storage media, or television broadcasting and communication. Motion Picture Experts Group (MPEG) in the International Electrotechnical Commission (IEC) of International Standardization Organization (ISO) has proceeded international standardization of motion picture coding method and already standardized several methods including ISO/IEC 13818-2 (MPEG2video).
Here a description is given, with reference to FIG. 9, of a conventional digital motion picture decoding apparatus for decoding the data coded by the MPEG coding method which is disclosed by Japanese Published Patent Application Number H8-32927, for example.
Reference numeral 900 designates a conventional digital motion picture decoding apparatus, numeral 911 designates a channel selecting means which takes coded video streams corresponding to plural programs as inputs and, in response to a program select signal input by a user, selects a bit stream corresponding to one of the plural programs to output the bit stream as coded picture data 901. This means is also configured to output a program change signal at the same time. The output coded picture data 901 passes through a code pattern detecting means 903 which is described later and is sequentially stored in an input buffer memory 902.
The code pattern detecting means 903 is a block which inhibits data from passing through to the input buffer memory 902 until a prescribed code pattern, I picture (intra-coded picture) of GOP (Group of pictures), is detected based on a change detection signal from the coded data change detecting means 904, and this means normally allows the data to pass through during the reproduction processing.
Reference numeral 905 designates a decoding means which makes a request for data to the input buffer memory 902, where the data obtained is subjected to decoding processing and then successively stored into a predetermined reproduced picture area in a reproduced picture memory 907.
A display picture deciding means 906 decides a reproduced picture to be displayed according to time information of each reproduced picture and the information about the data stored in the reproduced picture memory 907 obtained from a time information managing means 908 and a reproduced picture area managing means 909, outputting the picture as a reproduced picture data 910 for reproduction and display.
Next, a description is given of the operation for a transition from normal reproduction mode to special reproduction mode in which reproduction speed is changed to realize high-speed reproduction.
In response to a request for high-speed reproduction from a user, the coded picture data 901 is transmitted to the code pattern detecting means 903 in a format which comprises only an intra-frame coded picture (I picture) extracted from the bit stream recorded in record medium, and by detecting a pattern of contiguous I pictures, it is detected that the transition to special reproduction mode has been completed. Following this, sending data through to the input buffer memory 902 is stopped and a clear signal is output from the code pattern detecting means 903 to the input buffer memory 902, whereby the data stored in the input buffer memory 902 is erased.
When the code pattern detecting means 903 detects normal pattern again, cancel of the special reproduction mode is recognized and a blue-back processing which is described later is canceled, whereby sending data to the input buffer memory 902 is resumed.
Next, a description is given of the operation in the above-described configuration when the coded data in the process of reproduction becomes partially discontinuous due to change of program.
After detecting based on a program change detection signal that change of the coded data to be input has been performed, the coded data change detecting means 904 sends a clear signal to the input buffer memory 902 to clear all the data stored in the memory 902, while sending a data passing inhibit signal to the code pattern detecting means 903 in order to inhibit data from passing through to the input buffer memory 902 until a start code (I picture) of an intra-frame coded picture is detected by the code pattern detecting means 903.
At this time, blank pictures (Bu pictures) are inserted following a bidirectional predictive coded picture B3 (B picture), resulting in blue-back processing which provides a screen turned to blue.
Then, when the code pattern detecting means 903 detects the start code (I picture) of the intra-frame coded picture from the coded picture data after the change, passing data through to the input buffer memory 902 is permitted again and the blue-back processing is canceled, whereby reproduction operation is started again.
The decoding apparatus is configured to prevent reference picture error caused by connecting data arrays of different types at the change of coded data and avoid decoding the data left in the buffer at the change of the coded picture data, by performing the above-described processing.
The prior art digital motion picture decoding apparatus and the digital motion picture decoding method have the above-described construction which prevents reference picture error caused by connecting different types of data arrays at the change of coded data, but both the conventional decoding apparatus and the method have a problem that when the coded data is changed due to the change of program or other causes, blue-back screen is maintained during a prescribed period of time, making a viewer feel unconformable.
Moreover, in the case that change of coded data is not performed on a single piece of picture data basis and the coded data to be input is changed in the process of decoding a single piece of picture data by means of the decoding means 905, the operation of changing program in the middle of decoding processing as shown in FIG. 8(a) allows a situation as shown in FIG. 8(b) which is equal to the situation caused by inputting a data array to which a different type of data array is connected, resulting in decoding error and the considerably confused display resulted from error picture.
Furthermore, as shown in FIG. 5, since the structure of the data coded by a method employing bidirectional coding such as MPEG coding has an order different from both the order in which the picture data of the coded data is placed and the display order for reproducing pictures, forward predictive coded picture (P pictures), which is to be displayed following a sequence of bidirectional predictive coded pictures (B pictures), is decoded before decoding of the bidirectional predictive coded pictures (B pictures), and the resultant decoded data is stored in the reproduced picture memory 907. Therefore, as shown in FIG. 6), in the configuration only capable of clearing the data in the input buffer memory at the last stage, when the decoding processing has been already completed at the change of the coded picture data to be input and reproduced picture data which is not displayed yet (P6) is stored in the reproduced picture memory because the display time is not reached, that reproduced picture data is certainly displayed when the display time based on time information of the reproduced picture is reached. Therefore, with a long interval between each display time of reproduced pictures, an extended period of time is required before displaying the reproduced picture data which is unnecessary (P6) and it takes much time to change the coded picture data in the process of reproduction, whereby the blue-back screen is displayed for an extended period of time and makes a viewer feel uncomfortable.
An object of the invention is to provide a digital motion picture decoding apparatus and a digital motion picture decoding method which do not display blue-back screen during change of coded data due to change of program or the like, in order to prevent a viewer from feeling uncomfortable.
A further object of the invention is to provide a digital motion picture decoding apparatus and a digital motion picture decoding method which do not cause decoding error even when coded data is not changed on a single piece of picture data basis and the coded data to be input is changed when a single piece of picture data is being subjected to decoding processing.
A still further object of the invention is to provide a digital motion picture decoding apparatus and a digital motion picture decoding method which require a short time before displaying the reproduced picture data which is unnecessary and do not take much time to change the coded picture data in the process of reproduction, in the case that one piece of picture data, whose display time established according to time information on a display picture basis is not reached, has been already decoded at the change of coded picture data, and the reproduced picture is stored in the reproduced picture memory.
Other objects and advantages of the invention will become apparent from the detailed description that follows. The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the scope of the invention will be apparent to those of skill in the art from the detailed description.
According to a first aspect of the present invention, a digital motion picture decoding apparatus comprising an input buffer memory for storing coded data to be decoded, a reproduced picture memory for storing the decoded picture data to be displayed, and a display picture deciding means for deciding from the picture data stored in the reproduced picture memory a reproduced picture to be output, further comprises: a program changing means for changing the type of the coded data to be decoded; a program change detecting means for detecting from the output of the program changing means that the type of the coded data to be decoded is changed; and a display state maintaining means for controlling the picture data output from the reproduced picture memory so as to maintain the display state of the reproduced picture being currently displayed according to the decision of the display picture deciding means, when it is detected from the output of the program change detecting means that the change of the type of the coded data to be decoded makes the coded data not continuous in time sequence.
According to a second aspect of the present invention, in the digital motion picture decoding apparatus defined in the first aspect, the program change detecting means detects from an output of the program changing means a difference between a normal reproduction state and a reproduction state after change of program or a reproduction state at a reproduction rate different from the normal reproduction rate.
According to a third aspect of the present invention, the digital motion picture decoding apparatus defined in the first aspect further comprises an input buffer memory erasing means for erasing the coded data to be decoded which is stored therein, when the program change detecting means detects that the data subjected to decoding is not continuous in time sequence.
According to a fourth aspect of the present invention, a digital motion picture decoding apparatus comprising an input buffer memory for storing coded data to be decoded, a reproduced picture memory for storing decoded picture data to be displayed, and a display picture deciding means for deciding from the picture data stored in the reproduced picture memory a reproduced picture to be output, further comprises: a program changing means for changing the type of the coded data to be decoded; a program change detecting means for detecting from the output of the program changing means that the type of the coded data to be decoded is changed; a reproduced picture memory nullifying means for nullifying the data stored in the reproduced picture memory other than the data corresponding to the reproduced picture being currently displayed, when it is detected from the output of the program change detecting means that the change of the type of the coded data to be decoded makes the coded data not continuous in time sequence.
According to a fifth aspect of the present invention, the digital motion picture decoding apparatus defined in the fourth aspect comprises a reproduced picture area managing means for managing memory area information used when the decoding means writes the decoded picture data into the reproduced picture memory, and in this apparatus, the reproduced picture memory nullifying means erases the memory area information corresponding to the data stored in the reproduced picture area managing means except the data corresponding to a reproduced picture being currently displayed.
According to a sixth aspect of the present invention, in the digital motion picture decoding apparatus defined in the fourth aspect, the reproduced picture memory nullifying means erases the data in the reproduced picture memory except the data corresponding to the reproduced picture being currently displayed.
According to a seventh aspect of the present invention, in the digital motion picture decoding apparatus defined in the fourth aspect, picture data to be decoded next is written into a nullified memory area of the reproduced picture memory.
According to an eighth aspect of the present invention, a digital motion picture decoding apparatus comprising an input buffer memory for storing coded data to be decoded, a reproduced picture memory for storing decoded picture data to be displayed, and a display picture deciding means for deciding from the picture data stored in the reproduced picture memory a display picture to be output, further comprises: a program changing means for changing the type of the coded data to be decoded; a program change detecting means for detecting from the output of the program changing means that the type of the coded data to be decoded is changed; and a decoding stopping means for stopping decoding processing by the decoding means and controlling the decoding means such that the data being in the process of decoding at that time is written into the reproduced picture memory as the already decoded one, when it is detected from the output of the program change detecting means that the change of the type of the coded data to be decoded makes the coded data not continuous in time sequence.
According to a ninth aspect of the present invention, a digital motion picture decoding method comprises the steps of: detecting whether data is continuous in time sequence during decoding processing; and maintaining the display state of a reproduced picture being currently displayed when it is found in the first step that the data is not continuous in time sequence during the decoding processing.
According to a tenth aspect of the present invention, a digital motion picture decoding method comprises the steps of: detecting whether data is continuous in time sequence during decoding processing; and nullifying data other than the one corresponding to a reproduced picture being currently displayed, when it is judged in the first step that the data is not continuous in time sequence during the decoding processing.
According to an eleventh aspect of the present invention, the digital motion picture decoding method defined in the tenth aspect further comprises a step of stopping decoding processing and processing the data being currently decoded at that time as the already decoded one, when it is judged in the first step that the data is not continuous in time sequence during the decoding processing.